So as a new poster, I will add my observations in all of this, I am sure that it has been discussed at length, but now that the test has been made all the way to the US Supreme Court, the findings can be discussed with relative certainty at this time without a cloud of what may happen.
One cannot selectively interpret the laws, standards, and especially the intent of the laws. While there has been many successful cases of admiralty arrest of a shipwreck, that was never the intent of the laws, but simply to assure aid and provide incentive to provide assistance to ships in peril without liability.
The foundation of the laws is a ship in peril, so in reality, the test of a shipwreck fails at the foundation. While the US Courts have used Admiralty Arrest for shipwrecks, it really doesnt fit, and now with a test, may not have the same success in the future.
In the case of the Mercedes, the foundation, once again, with sovereignty, and commercial vs non-commercial, status failed at the foundation. The definition was selectively narrowed to a state owned vessel on commercial mission do not have protection. What was selectively left out was the definition as a warship. A States warships are always afforded sovereignty, hence the warship and site status, were correctly determined from the beginning by the US Courts.
What was also veiled, was the location of the site, as international laws include a States rights to underwater resources, to its respective EEZ.
As part of NATO, a Spanish warship can board a vessel in international waters as well.